Great importance has been attached to the study of the toxicity of silica, mainly the involvement of silica in silicosis, but there has been little work concerned with the effect of silicon in normal metabolism and until recently there has been no proof that silicon plays any definite role in vital processes. Silicon has generally been considered to be nonessential. It was recently demonstrated in this laboratory that silicon is required for normal growth and development in the chick when a low silicon diet is fed in a trace element controlled environment. The effect on growth is marked suggesting that silicon must play an important physiological role in the body. The objectives of this reseach are to establish the physiological role of silicon, that is to establish the function, site and mode of action of the element with special attention to its possible role in connective tissue metabolism and with additional emphasis on the function of silicon in the aorta and its relationship to the other components of the aortic wall. In vivo studies using chicks and rats and employing the conditions developed in this laboratory to produce a silicon deficiency in the young animal and in vitro studies including isolation and quantitative analyses of connective tissue components, electron microprobe and tissue culture studies will be carried out to attain these objectives.